It is increasingly important that aircraft turbine engines have a long life. Increased operating costs, e.g. fuel price escalation, enhance the importance of engine life consideration and materially reduce the desirability of continuing to use old engines even though they are functionally sound. However, the cost of replacing such engines is quite high and, therefore, impractical.
Among the factors which have improved the efficiency of new technology engines is the ability of the turbine alloys used in these engines to operate at higher temperatures and stress. This increased efficiency results significantly from the substitution of single crystal super alloys for equiaxe super alloys. However, the new single crystal alloys are generally more dense, with such components, therefore, generating centrifugal forces which potentially exceed design limitations of some of the older engines in service.
The structures of some of the older turbine engines, including turbine discs, shafts, bearings and supports are not capable of withstanding the centrifugal forces of the newer, more dense high performance single crystal super alloys at the speeds which make the use of such alloys advantageous. Thus, the speed of the engines must be reduced, thereby negating the potential efficiency upgrading which would otherwise justify rebuilding such engines. With older engines, this tends to further reduce the desirability of rebuilding them. In the case of engines of more recent design, this approach can negate or largely negate the engine's potential for functional improvement.